Actuators are used in disk-based information storage and retrieval to position the read/write elements over a radial location on a spinning disk-shaped storage element or elements. The actuator generally has a rotary actuator arm or parallel arms, solidly connected through a pivot point to a coil or a bobbin on the other side of said pivot point. The coil is positioned between or around permanent magnets. Energization of the coil causes it to move in a well known way between the magnets. This movement through said pivot point causes the read/write heads or elements to be moved in a direction opposite to said coil and bobbin, radially over the surface of the disk media.
Rotary actuator design is optimized in terms of mass and resistance when using a center pole which is inside the moveable coil and permanent magnets are outside the coil. The problem with this design is that a steel center pole increases the inductance of the coil slowing the coil's ability to energize and respond by accelerating or decelerating in its direction of movement. The prior art, in response to this problem, wrapped a shorted turn of copper around the center pole which would develop an induced current responsive to the reactance flux of the coil. Using a shorted copper turn however did not lower the inductance of the coil to the degree this invention does. Use of the shorted copper turn also requires an increase in the steel to magnet gap thereby lowering the torque available per amp of coil energization current.
Examples of solutions dissimilar to this invention for similar problems with inductance and reactive flux in voice coil or linear actuators include the shorted turn in U.S. Pat. No. 3,521,092; a conductive ring, coextensive with the winding in U.S. Pat. No. 3,176,170; use of a double set of permanent magnets to prevent reactance flux from extending through the entire coil path as in U.S Pat. No. 3,599,020; use of "sensing" coils responsive to drive coil energization in U.S. Pat. No. 3,470,399; and also the "bucking" coil of U.S. Pat. No. 3,619,673.
For various manufacturing reasons explained in more detail within, none of these solutions provide the same minimization of reactance flux in the center pole nor do they minimize the inductance of the coil as effectively as this invention.